1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to an electrostatic discharge protection element in a semiconductor device employing an SOI, i.e., silicon on insulator, structure.
2. Related Arts
MOSFET research forming a monocrystalline silicon layer on an insulator has been performed amidst advances in faster speed and greater integration of semiconductor devices. In a semiconductor device of this type, normally, to prevent MOSFETs, particularly gate-oxide films, from being destroyed by excessive current due to static electricity, surges, or the like, an electrostatic discharge protection element employing a diode is required on an input pad portion, as is indicated by Japanese Patent Application Laid-open No. 2-97066 Patent Gazette.
Here, in a protective diode of this type, a pn diode of large area is formed at a device formed on a bulk-silicon substrate by a pn junction of an n.sup.- well and p.sup.+ diffusion layer or a p.sup.- well and an n.sup.+ diffusion layer, and large current flow is possible.
In an element formed on a thin-film SOI substrate, however, a pn junction can be formed only at a lateral cross-section of the thin SOI layer, and so an attempt to gain a pn-junction area equal to the foregoing bulk requires an element area several tens of times larger, and the problem of impracticality exists.